1. Field of the Invention
This invention relates to a method and apparatus for drilling gas relief holes through coal beds, and more particularly to a method and apparatus for maintaining a rotating drill bit along a trajectory parallel to the bedding planes of subterranean coal beds.
The use of rotary drilling to form long horizontal gas relief holes in coal beds is known in the art as a means for degasifying a coal bed in advance of mining. These gas relief holes are either vented or connected to a vacuum source to remove methane from the coal bed. The greatest problem encountered in drilling these gas relief holes is that of maintaining the bit trajectory parallel to the coal bed such that the resulting holes are actually through the coal bed rather than through an overlying or underlying formation.
2. Description of the Prior Art
The state of the art to which the present invention pertains is set forth in detail in a Bureau of Mines Report of Investigations published in 1975 numbered 8097 and entitled "Rotary Drilling Holes in Coalbeds for Degasification", by Cervick et al, available in the U.S. Department of the Interior Library. That report describes the use of rotary drill bits attached to drill rods and maintained in a desired trajectory by a combination of bit thrust, rotational speed and drill rod centralizer spacing. That report further notes that locating a centralizer or stabilizer near the drill bit will cause a slight upward trajectory to the bit with proper drill thrust and bit rotational speed, and further notes that a downward trajectory can be obtained by locating a centralizer several meters behind the bit. However, relocating a centralizer to facilitate a change in bit trajectory due to change in bed dip or to the bit straying out of the coal bed for any reason has previously involved removal of the entire drill string. Such a procedure is time consuming and unproductive. The use of two centralizers as suggested in the report also presents problems.
U.S. Pat. No. 3,131,778 describes a device for changing the direction of a hole being drilled, the device including a housing having a shaped slot with lateral offsets. A drill bit is connected to the housing by a pin movable in the slot. However, the apparatus described in that patent is designed for drilling generally vertical boreholes, and is not suitable for drilling generally horizontal gas relief holes in coal beds.
There has been a continuing need, prior to the present invention, for an improved method and apparatus for controlling the trajectory of horizontal gas relief holes in coal beds.